This invention relates to a process for providing a variable anodic thermal control coating to aluminum surfaces for use as the external surface area of space vehicles to passively control the temperature of the vehicle when exposed to a spatial environment.
The most significant parameter that can be varied to control the temperature of satellites and space vehicles is the ratio of the solar absorptance to the low temperature emittance (.alpha..sub.s /.epsilon..sub.T) of the external vehicle surface area. The expression .alpha..sub.s /.epsilon..sub.T is the ratio of the absorptivity of the face of a plate to solar radiation (.alpha..sub.s) to the emissivity of the face of the plate to thermal radiation (.epsilon..sub.T). Since these quantities are dependent only on the unit surface of an object, the temperature of the object can be adjusted to the desired value by selecting a coating for the object's face that has the requisite value of .alpha..sub.s /.epsilon..sub.T. This is the routine procedure used in the design of spacecraft.
Previous passive methods of controlling the surface temperature of spacecraft have included surface oxidation, vapor deposition of thin metallic films, conversion coatings, dielectric films, and partially coating the surface area of the spacecraft with paint to attain the desired effective thermal radiation characteristics. The disadvantages of these prior art methods include the numerous problems included in maintaining the required delicate environment for adequate application thereof and none had the capability of changing different variables in the process to select the desired values for thermal emittance and solar absorptance. The disadvantages of paints is that most of those presently available do not possess stable thermal radiation characteristics when exposed to the space environment. Further, no known method of paint application has been developed capable of obtaining a wide range of thermal radiation characteristics or for providing a complete coverage of the surface area to minimize thermal gradients.
It is therefore an object of the present invention to provide a coating process for aluminum surfaces that can be controlled to produce the desired thermal emittance and solar absorptance value within the respective ranges of 0.10 to 0.72 and 0.2 to 0.4.
Another object of the present invention is a novel process for applying an anodized coating to an aluminum surface to produce consistent thermal control coating characteristics to the surface.
Another object of the present invention is an anodizing process for coating aluminum surfaces that is sensitive to the parameters of voltage, rate of voltage application, time, temperature, acid concentration, material pretreatment and sealing.
An additional object of the present invention is a novel process for providing a variable anodic thermal control on aluminum that permits selection of the thermal emittance and solar absorptance parameters independent of each other.
According to the present invention the foregoing and additional objects are obtained by a three phase process involving initial material processing, anodizing the material and post material processing. The process described herein is applicable for any aluminum or aluminum alloy in plate, sheet or foil configuration.
The purpose of the initial material processing is to prepare the material for anodizing and to establish an initial value of thermal emittance (.epsilon..sub.T) and solar absorptivity (.alpha..sub.s). This preparation involves cleaning the aluminum material selected by immersion in a metal cleaning bath such as MIL-M-7752 metal cleaner at 5 Av oz/gal balanced with water and operating at 160.degree. F. to 200.degree. F. (nominal 180.degree. F.) for 5-10 minutes (nominal 8 minutes). The aluminum material is then removed from the cleaning solution, rinsed thoroughly in a water bath at ambient temperature and then immersed in a deoxidizer solution. The deoxidizer solution employed in the present process was a mixture of chromic acid (federal specification O-C-303 Type II at 5 Av oz/gal) and sulfric acid (federal specification O-5-809 at 12 Fl. oz/gal) balanced with water and employed at 150.degree. F. to 180.degree. F. (nominal 170.degree. F.) for 2-5 minutes (nominal 4 minutes). The deoxidized aluminum surface material is again rinsed in a water bath at ambient temperature while physically agitating the aluminum surface to ensure that all particulates are removed from the surface. After drying by filtered forced air, the initial thermal emissivity (.epsilon..sub.T) and solar absorptivity (.alpha..sub.s) values for the material are established by conventional measuring procedures.
After establishing the initial thermal emission and solar absorptance values, the aluminum surface is anodized by immersing in a chromic acid solution containing CrO.sub.3 in the range of 3 to 10 percent by weight balanced with water. A D.C. voltage is applied between the aluminum and the chromic acid solution by a conventional process. Starting at zero volts, the voltage is increased at a predetermined rate (notminal 30 seconds) up to a selected voltage and maintained for a selected period of time. The rate, voltage and time, along with the parameters of initial thermal emittance (.epsilon..sub.T) and solar absorptance (.alpha..sub.s), the temperature of the chromic acid solution, acid concentration of the solution and the material to be anodized all combine to determine the final value of .epsilon..sub.T and .alpha..sub.s.
The aluminum is then removed from the chromic acid bath and rinsed with water to remove the residue chromic acid. Sealing of the coating is then attained by placing the aluminum surface material in a clear water sealing bath at a temperature of 170.degree. F. to 200.degree. F. (nominal 180.degree. F.) for ten minutes. The aluminum is then dried using filtered forced air at ambient temperature and final .epsilon..sub.T and .alpha..sub.s values measured, as before.